1. Field of the Invention
The present invention relates to an electroplating cell, and a method of forming a metal coating, and more specifically relates to an electroplating cell which is capable of easily forming a metal coating on a surface of a cathode (plated object), and a method of forming a metal coating using the electroplating cell.
2. Description of Related Art
A technique of forming a pattern formed of a metal coating (hereinafter, referred to as “metal pattern”) on a conductive substrate with a simple method is required. A technique of masking a portion other than a metal pattern to perform wet electroplating is most commonly used. However, in this technique, a mask forming step and a mask removing step are required, and there is a problem in that the cost for the management and waste liquid treatment of a plating solution is high. Recently, a method of forming a metal coating with a “physical method” such as physical vapor deposition or sputtering not having the above-described problem and then removing a masking portion has been adopted. However, in this method of physically forming a metal coating, a film forming speed is generally slow, and a vacuum unit is necessary. Therefore, it is difficult to say that a system using this method is an economical high-speed production system.
On the other hand, as another method in which masking is not necessary, a method of coating a substrate with an ink, in which conductive fine powder and a binder are mixed, using a “printing method” such as screen printing or an ink jet and then removing the binder by firing has also been adopted. However, with this “printing method”, it is difficult to form a circuit having low volume resistivity even if a volatile or sublimable binder is used.
However, recently, as an attempt during electroplating to prevent electrodeposition at a portion other than a target portion and to form a circuit without masking, a gel electrolyte (Japanese Patent Application Publication No. 2005-248319 (JP 2005-248319 A)) and a cation exchange membrane (Japanese Patent Application Publication No. 2012-219362 (JP 2012-219362 A) and International Publication WO 2013/125643) have been proposed.
When such a separator is used, a current density of approximately 10 mA/cm2 is obtained at room temperature, for example, in Cu plating in which electrodeposition from an aqueous solution is relatively easy. However, in order to perform a film forming process (high current density electrodeposition) at a higher speed than that of the Cu plating, it is necessary to take an action, for example, to increase a metal ion concentration or to increase the temperature. Therefore, a higher cost is required. In particular, it is difficult to electrodeposit metal (for example, metal in which deposition potential of nickel ions, zinc ions, tin ions, or the like is low), in which an electrodeposition reaction (reduction deposition reaction) competes with a hydrogen ion discharge reaction (hydrogen evolution reaction), from an acidic or slightly acidic aqueous solution having a high hydrogen ion concentration using a separator.
The details of the reason for this phenomenon are unclear, but it is considered that this phenomenon is caused by the following reasons (1) to (3).
(1) Hydrogen is produced at an electrodeposition portion, and defects (voids) are formed.
(2) Due to deposition over voltage being too low, metal is electroplated in a fine powder form or in a lump, and when the electrodeposition is performed in a state where a separator and a cathode are in close contact with each other, an electrodeposit infiltrates into the separator.
(3) Due to a pH increase at a cathode interface caused by hydrogen production, a hydroxide is produced at an electrodeposition portion, and passivation (increase in bath voltage) progresses.
During electrodeposition using an insoluble anode and a separator, hydrogen ions produced from an anode chamber solution are blocked due to the presence of the separator, and thus the pH at a cathode interface is likely to increase. Therefore, the above-described problems are particularly severe. In particular, in an electroplating cell (not containing a cathode chamber solution) in which a separator and a cathode are in close contact with each other, or in an electroplating cell in which the amount of a cathode chamber solution is extremely small, even the amount of hydrogen produced by hydrogen evolution reaction is extremely small, it is difficult to perform normal electrodeposition due to the effects of (1) and (2) described above.